1. Field of the Invention
The present invention relates to an organic nonlinear optical material which can be used in a highly efficient wavelength conversion device for use in optical communication wavelength bands, to 2-adamantylamino-5-nitropyridine which is suitable for use in the optical material, and to a production method thereof.
2. Description of the Prior Art
As well known, organic nonlinear optical materials are drawing attention as materials for achieving highly efficient wavelength conversion devices. 2-Adamantylamino-5-nitropyridine (hereinafter abbreviated to as AANP) is known to be useful as a material of a wavelength conversion device because the compound has a large second order nonlinear optical coefficient (refer to Japanese Patent Application No. 282221/1987 [Japanese Patent Application Laid-open Publication No. 124834/1989]). Since AANP has wavelength conversion characteristics particularly in 1.3 to 1.6 .mu.m wavelength region, which are optical communications wavelength region, application of this compound is being investigated for measuring instruments utilizing wavelength conversion in optical communications wavelength region.
Main usable wavelength region of AANP are at and around 1.32 .mu.m wavelength and at and around 1.55 .mu.m wavelength. At and around 1.32 .mu.m wavelength, a bulk crystal of AANP has a very high wavelength conversion efficiency of 3.times.10.sup.-3 (/W) for second harmonic generation (SHG). On the other hand, at and around 1.55 .mu.m wavelength, the same AANP bulk crystal has only a small wavelength conversion efficiency of 2.times.10.sup.-4 (/W) for SHG as compared with the value at and around 1.32 .mu.m wavelength. Therefore, a nearly the same wavelength conversion efficiency at and around 1.32 .mu.m wavelength has been required for AANP also at and around 1.55 .mu.m wavelength.
However, no solution for preventing the reduction in wavelength conversion efficiency at and around 1.55 .mu.m wavelength of the AANP has been found up to now, because the cause of the reduction has not been revealed.